The selective catalytic reduction (SCR) of nitrogen oxides (NOx) in flue gases is used in many industries worldwide to comply with national and international emission legislation. Nitrogen oxides formed in the combustion process of fossil and renewable fuels are reduced with a reductant, such as ammonia, on a catalytic surface. Various catalysts have been used on a variety of substrates, such as vanadium oxides, ion-exchanged zeolites etc. The catalysts can be prepared in different formulations and can be present in different forms, such as extruded or coated honeycombs, metal substrates etc. One of the major factors that determine the selection of the appropriate catalyst is the temperature of the flue gas. While ammonia is preferred as a reductant, the direct use of ammonia is problematic due to the hazardous nature of gaseous ammonia. Therefore, substances that are easy to handle and decompose to form ammonia when injected into the hot flue gases are normally used. For example, an aqueous urea solution decomposes at temperatures above 140° C. to form ammonia and isocyanic acid (HNCO), which then hydrolyzes to form ammonia and carbon dioxide. However, the generation of ammonia from an aqueous urea solution is a relatively slow process. If the residence time of urea in the hot gas stream is too short, this can lead to precipitation on the reactor walls or worse, on the catalyst. Therefore, relatively long injection ducts with a length of several meters are located upstream of the actual catalyst used in current state of the art SCR applications. These long ducts are typically a straight tube through which the exhaust flows and in which the reductant is injected into the hot gas stream by means of an injector or a lance.
The SCR systems described above have generally been used on large, stationary systems, such as power plants. Smaller SCR systems have been used in automotive applications and in engines generally below 600 kW. These smaller SCR systems have different designs due to lower exhaust volumes and therefore, a smaller mass flow of reductant needed to be introduced into the system. Recently, emission regulations for the 500 to 4500 Kilowatt (kW) diesel and gas engines have been established for the marine, off-road and power generation sectors. Currently the systems used in engines of these sizes consist of a long exhaust pipe (up to around 10 m) with a large diameter (up to around 0.6 m) and a SCR reactor located in the flow of the exhaust gas. An aqueous urea solution is injected directly in the exhaust gas by means of a lance. The urea subsequently converts to ammonia in the full exhaust gas flow. To achieve a uniform ammonia concentration pattern across the catalyst cross section, the flow is deliberately disturbed by static mixers. Often, the ammonia is directly introduced through an ammonia injection grid (AIG) into the full exhaust flow before being passed through one or multiple mixers and then the SCR catalyst. Uneven flow distributions can result in spots with low temperature sections leading to precipitation or corrosion from the partly decomposed urea. These urea losses also result in a decrease in NOx conversion activity because precipitated material cannot participate in the reaction to convert urea to ammonia.
Space is a crucial factor in applications for marine, off-road and power generation sectors and the use of space can affect the economics of operation in these sectors. For example, a super yacht or a ferry might lose passenger space directly resulting in lost income. Large mining excavators and trucks would need to reduce the loads that they can move or carry, resulting is the need to perform additional excavations or make additional trips in order to move the same amount of material. In certain vehicles, such as tug boats, the machine rooms may not have the space required to install the current state of the art SCR set-up.
The compact SCR system described herein allows for the use of urea in reducing the levels of nitrogen oxides (NOx) in exhaust gases using an SCR process in engines having a size in which exhaust aftertreatment system space constraints had previously been an obstacle to their use. One of the advantages of the compact SCR system described herein is that the system, in addition to being able to be used with new engines in the sectors described above, also allows for the installation of aftermarket systems so that existing engines will be able to reduce their emissions as well.